1. Field of the Invention
The present invention relates to a display apparatus, such as a cathode-ray tube (CRT) with a deflection yoke attached thereto, and more particularly, to a resistive primer coating that forms a barrier between the tube and the yoke.
2. Description of the Background Art
A display apparatus, such as a television picture tube or a cathode-ray tube (CRT), typically comprises a faceplate panel and a tubular neck connected by a rectangular funnel. A three-color luminescent screen is disposed on an interior surface of the faceplate panel. An electron gun is disposed within the tubular neck and generates at least one, and preferably three electron beams that are directed toward the three-color luminescent screen.
The interior surface of the rectangular funnel is coated with a first conductive coating that extends from the electron gun to the luminescent screen, while a portion of the exterior surface of the funnel is covered with a second conductive coating. The conductive layer on the exterior of the funnel is grounded and, together with the interior conductive layer, forms a capacitor that serves to smooth the voltage to the electron gun. A system of deflection coils, called a deflection yoke, is disposed around the neck-funnel transition region of the tube.
A high resistance coating or layer is typically provided on an exterior portion of the tube underneath the deflection yoke, in order to reduce interference generated by the deflection system. The high resistance layer typically has a resistance between about 106 ohm/square to about 1010 ohm/square and is electrically connected to the second conductive layer formed on portions of the exterior surface of the tube.
Apparatuses made with an insulating coating, or having no coating at all underneath the yoke, may develop an inductive charge which xe2x80x9cflashes overxe2x80x9d, or electrically discharges, to the deflection yoke or to the second conductive layer formed on portions of the exterior surface of the funnel. This problem is exacerbated in the current generation of high resolution display tubes in which the horizontal scan rate is twice that of prior tubes, thus requiring the deflection yoke to operate at a higher current, thereby increasing the magnitude of the induced charge as well as generating additional heat underneath the yoke.
The high resistance coating underneath the deflection yoke must also be compatible with the manufacturing processes used to make the display tube, such as the alignment and attachment of the deflection yoke. In current tube manufacturing processes, the yoke, after alignment, is secured to the tube with an adhesive. Conventional high resistance coatings comprised of polychloroprene may be incompatible with yoke attachment since their high electrical resistance tends to be diminished when heated to the temperatures used to cure many adhesives. Also, the bond strength of polychloroprene resistive coatings with many hot melt adhesives used to anchor the yoke to the high resistive coating is poor in high humidity.
Thus, a need exists for a high resistance coating which is compatible with the above-described yoke attachment process, and which provides the protection needed to reduce electrical interference from the yoke and to prevent the build-up and flashover of induced charge.
The present invention relates to a display apparatus comprising an evacuated envelope having an interior surface and an exterior surface. The envelope includes a faceplate having a luminescent screen on the interior surface thereof. An electron gun is disposed within a neck of the envelope for generating at least one electron beam that is directed toward the luminescent screen. A funnel connects the neck of the envelope with the faceplate. A deflection yoke is disposed around the envelope at the junction of the neck and the funnel. The yoke is attached to the envelope with an adhesive.
A resistive primer coating is disposed on the envelope and forms a barrier between the yoke adhesive and the envelope. The resistive primer coating is formed from an aqueous formulation comprising a copolymer of chloroprene and methacrylic acid, a conductive material and at least one cross-linking agent. Cross-linking the acid moieties of this copolymer provides a resistive primer coating with superior pull strength in high humidity and whose electrical resistance is relatively insensitive to cure temperature conditions.